This invention relates to a process and an apparatus for scrubbing of NO.sub.x and/or SO.sub.2 from gas mixtures containing NO.sub.x and/or SO.sub.2 with a scrubbing agent containing an iron(II) salt and solvent.
Air pollution, especially egregious during episodes of smog, has its origin mainly from the exhaust gases from fossil fuel power plants, other large-scale industrial facilities (e.g., iron and steel plants, petroleum refineries) and automobile exhaust gases. The main components of the air pollution are CO, SO.sub.2, hydrocarbons, NO.sub.x and suspended particles. (SO.sub.2 together with suspended particles is often considered as an indicator for the entire air pollution, and to appreciate the scope of the problem, it is noted that SO.sub.2 emission by burning of sulfur-containing fossil fuels in the Federal Republic of Germany is estimated at several million tons yearly.) SO.sub.2 is easily soluble in water and above all acts as an irritant to the mucous membranes of the upper and middle respiratory tract and of the eyes. NO.sub.x is formed from the elements in combustion processes. NO in the air is generally quickly oxidized to NO.sub.2, which because of its high water solubility is resorbed in the lung alveoli and causes an irritating effect. In addition, any residual NO is considered to be an extraordinarily harmful air pollutant, because under the effect of ultraviolet radiation it can react with hydrocarbons, CO and water vapor, as they occur in automobile exhaust gases, to form photochemical oxidation products, especially ozone. Particularly harmful photochemical reaction products are organic peroxides and nitric acid compounds of peracetic acid, which have a considerable toxicity for the respiratory tract and which damage goods and vegetation.
Therefore, numerous known processes have been proposed, which aim at removing from flue gases SO.sub.2, NO.sub.x or NO, alone or in combination, to prevent these materials from escaping into the atmosphere. In these efforts to solve the problem, the so-called dry and so-called wet processes are distinguished. In the case of wet processes, acid and alkali processes are distinguished. In the acid processes, the waste gases are scrubbed with cold nitric acid and the absorbed nitrogen oxides are fed back into the absorption system or catalytically oxidized by heating and/or air stripping. In the alkali processes, the waste gases are scrubbed with aqueous solutions of sodium, calcium or magnesium hydroxides or carbonates. However, these processes have not yet been able to provide complete satisfaction since they can be used only at high pressures of only at low pressures [(Ullmanns Encyklopaedie der technischen Chemie (Ullmanns Encyclopedia of Industrial Chemistry), 4th edition, volume 20, page 328)].
More recently a process has become known (U.S. Pat. No. 4,418,044, Kulik) which makes it possible to remove NO.sub.x and SO.sub.2 simultaneously from waste gases, flue gases, etc. In this known process the operation is performed with solutions of iron(II) ions and S.sub.2 O.sub.3.sup.-- ions. The iron (II)ions as a rule are added in the form of sulfates, the S.sub.2 O.sub.3.sup.-- ions in the form of potassium or ammonium salts. As solvent for these substances, water is primarily used, but to which specific amounts of nonaqueous solvents, e.g., methanol or glycol, are added to increase the absorption of NO.
Kulik refers to the use of a "nonaqueous scrubbing solution". However, it is clear from the disclosure that by the term "nonaqueous" is meant an aqueous solution to which nonaqueous liquids have been added, e.g., alcohols. In column 3, lines 5-12, Kulik discloses that the alcohol in the scrubbing solution provides an "anhydrous environment" which improves the stability of the NO complex and that other "nonaqueous liquids" can be used "in combination with a ferrous thiosulfate scrubbing solution." In example 1 of Kulik, a scrubbing solution comprising a total of 240 mol containing 40 mol of methanol was used.
Moreover, this known process employs a molar ratio of S.sub.2 O.sub.3.sup.-- /Fe.sup.++ of about 5.0. Thus, this known process has the disadvantage of a very high consumption of chemicals. Thus, e.g., per mol of absorbed NO (30 g) 190 g of K.sub.2 S.sub.2 O.sub.3 and 49 g of H.sub.2 SO.sub.4 are consumed. Moreover, the known scrubbing agent apparently cannot be regenerated easily, if at all.